Ammonium nitrate, which according to the Lebanese authorities caused the, is an odorless crystalline substance commonly used as a fertilizer and has caused numerous industrial explosions over the decades. These notably include one at a Texas fertilizer plant in 2013 that killed 15 people and was , a North Korean railway explosion that killed 161 people in 2004 and the Oklahoma City bombing in 1995.
When combined with fuel oils, ammonium nitrate creates a powerful explosive widely used in the construction industry, but also by rebel groups such as the Taliban for improvised explosives.
Two tons were used to create the bomb inwhich destroyed a federal building, killing 168 people.
Lebanese Prime Minister Hassan Diab said 2,750 tons of ammonium nitrate that had been stored for years in a Beirut port warehouse had exploded.and causing widespread damage to the capital.
Ammonium nitrate was among the chemicals stored in a warehouse in the northern China city of Tianjin that ignited in 2015, triggering an explosion that killed at least 165 people and caused more than $ 1 billion in damage.
An investigation found that improper storage of chemicals was to blame.
In agriculture, ammonium nitrate fertilizer is applied in the form of granules and dissolves quickly under moisture, allowing nitrogen, which is key to plant growth, to be released into the soil.
However, under normal storage conditions and without very high heat, it is difficult to ignite ammonium nitrate, Jimmie Oxley, professor of chemistry at the University of Rhode Island, told AFP.
“If you watch the video (of the Beirut explosion), you saw the black smoke, you saw the red smoke – that was an incomplete reaction,” he said.
“I suppose there was a small explosion that caused the ammonium nitrate to react – whether that small explosion was an accident or something on purpose that I haven’t heard yet.”
This is because ammonium nitrate is an oxidant: it intensifies combustion and allows other substances to ignite more quickly, but by itself it is not very combustible.
For these reasons, there are generally very strict rules on where it can be stored: for example, it must be kept away from fuels and heat sources.
In fact, many EU countries require calcium carbonate to be added to ammonium nitrate to create calcium ammonium nitrate, which is safer.
In the United States, regulations were tightened significantly after the Oklahoma City attack.
According to anti-terrorism standards for chemical plants, for example, plants that store more than 2,000 pounds (900 kilograms) of ammonium nitrate are subject to inspections.
Despite its dangers, Oxley said the legitimate uses of ammonium nitrate in agriculture and construction have made it indispensable.
“We wouldn’t have this modern world without explosives and we couldn’t feed the population we have today without ammonium nitrate fertilizer,” he said.
“We need ammonium nitrate, we just have to pay close attention to what we do with it.”